1. Field of the Invention
The present invention relates to a silver-silver chloride electrode. More particularly, it relates to a silver-silver chloride skin electrode adapted to a skin of a living-body for detecting electrical signal generated by biopotential phenomena such as electrocardiogram, electromyogram and electroencephalogram to lead the electrical signal to a monitoring equipment.
2. Description of the Prior Arts
When a biopotential phenomenon is measured by the electrocardiogram, and the electroencephalogram etc., the bipotentials are very weak, for example, on the order of several mV. Thus, in order to detect precisely the biopotentials and to lead it to a monitoring equipment, the skin electrode adapted to a skin of a living-body should have stable electrode potentials, small electrode impedance and should not cause noise potentials. Thus, the skin electrode should impart highly reversible electrode reaction as a nonpolarized electrode.
It has been known that a silver-silver chloride electrode is optimum as the biopotential nonpolarized electrode.
It has been reported that the silver-silver chloride electrode having excellent characteristics as a skin electrode is prepared by electrolyzing a silver plate as an anode in an aqueous solution of sodium chloride to form a film of silver chloride on the surface of the silver plate or by press-molding a mixture of powdery silver and powdery silver chloride. However, these processes have various disadvantages that many steps are required and a cost for equipment is expensive and a cost for raw materials is expensive. Such silver-silver chloride electrode is not suitable as a disposable electrode. Thus, the silver-silver chloride electrode has not been widely used.
In order to overcome these disadvantages on the conventional preparation of the silver-silver chloride electrode, it has been proposed to prepare a silver-silver chloride electrode by coating a layer of a homogeneous mixture of silver grain, silver chloride grain and a synthetic resin such as polyvinyl chloride and epoxy resin on a substrate for the electrode in Japanese Unexamined Patent Publication No. 24195/1972. It has been also proposed to prepare a silver-silver chloride electrode by coating on a conductive substrate, a layer having an organic matrix made of epoxy resin, etc. in which silver chloride grain and silver grain or silver coated grain coating silver on a core such as ceramic, silica or glass grain, are dispersed, in Japanese Unexamined Patent Publication No. 26789/1976. It has been also proposed to prepare a silver-silver chloride electrode by bonding silver grain and silver chloride grain with an epoxy resin adhesive on a metal surface such as silver, in Japanese Unexamined Patent Publication No. 59488/1976.
These silver-silver chloride electrodes are prepared by forming the coated layer on the substrate whereby the disadvantages of the conventional processes are not found and stable electrode potential (off-set voltage) characteristic is given and noise and fluctuation of base line are small. However, medical requirements such as surgical operations with a electric knife in monitoring an electrocardiograph have been increased and large input noise may be applied together with the biopotentials in many cases. The large input noise may cause the voltage on the order of 1 Volt or the current on the order of several mA. When such large input noise is applied, high polarization is caused by the known silver-silver chloride electrode so that it is not practically used to be inoperative.
The inventors have studied to develop a stable silver-silver chloride electrode which is easily prepared and economical and is suitable as disposable skin electrode and has stable off-set voltage characteristic and does not cause substantial noise and fluctuation of the base line and maintains non-polarizability during the input of large input noise caused by an electric knife, etc.
The inventors have found and proposed an improvement of the conventional silver-silver chloride electrode having a coated layer of silver grain, silver chloride grain and an organic matrix on a conductive substrate by substituting the silver grain by carbon grain coated with silver so that the skin electrode is easily prepared and has stable off-set voltage characteristic and has improved non-polarizability under the unpredictable large input noise.
The skin electrode has excellent characteristics as illustrated. However, as the conventional technological problems, the layer of silver grain, silver chloride grain and an organic polymer as the matrix is bonded on the substrate so that there is the disadvantage of relatively weak adhesive strength between the coated layer and the substrate. Even though, the skin electrode is disposable, high adhesive strength between the coated layer and the substrate is required for the silver-silver chloride electrode. In order to satisfy the requirement on the adhesive strength, a ratio of the organic matrix should be high so as to cause inferior electric characteristic of the skin electrode. On the other hand, in order to impart excellent electric characteristics, the ratio of the organic matrix should be decreased so as to cause inferior adhesive strength, higher defect rate in the production or inferior storagable characteristic of the skin electrode. In measurements by using the skin electrode, an electroconductive cream or jelly containing an electrolyte is placed between the skin electrode and the skin so as to decrease the contact-resistance and the fluctuation of the contact resistance. In the case of the disposable skin electrode, the cream or the jelly is immersed in sponge and the sponge is placed on the coated layer of the skin electrode held in a casing. When the disposable skin electrode is stored for a long time, the coated layer is deteriorated by the component of the electroconductive cream or jelly, so as to deteriorate the electric characteristics and the adhesive strength.